A selective catalytic reduction method (SCR method) refers to a selective chemical reaction in which only the nitrogen oxides are reduced instead of all the exhaust gas components of the exhaust. In automotive engineering, this method is increasingly being used in trucks and commercial vehicles having diesel engines to make it possible to reduce pollutant emissions to the extent that they comply with the EU4 standard. The ammonia required for the reduction is not added directly, i.e., in pure form, but instead is added in the form of an aqueous urea solution which is injected into the exhaust line where it triggers cleavage of the urea-water solution into ammonia and water by means of a hydrolysis reaction. The advantage of the SCR method is the removal of soot particles and nitrogen oxides from the exhaust and therefore definitely reduced emissions. The disadvantage of the previous method is in particular the fact that mixing of urea with the exhaust to be cleaned cannot be adequately accomplished.